1. Technical Field of the Invention
The present invention relates generally to a wave-shaping circuit for converting an input alternating signal into a pulse signal, and more particularly to a wave-shaping circuit with a feedback circuit designed to compensate for variation in central voltage level of an input alternating signal.
2. Background Art
A wave-shaping circuit is usually used for processing an alternating signal produced according to rotation of a wheel of an automotive vehicle to determine a wheel speed. There has been proposed a wave-shaping circuit which converts through a pull-up resistor an alternating signal outputted from a speed sensor into another alternating signal varying across a given central voltage level and compares it with a preselected constant threshold level through a comparator to produce a pulse signal. However, when the center of amplitude of an output signal from the sensor varies due to some factor, the simple comparison, as discussed above, with the constant threshold level encounters a difficulty in that the input alternating signal is not wave-shaped precisely.
For avoiding the above drawback, Japanese Patent First Publication No. 62-135773 discloses an improved wave-shaping circuit, as shown in FIG. 6, which sets a threshold level of a comparator so as to follow the variation in central level of amplitude of an output signal from a sensor.
The wave-shaping circuit shown in FIG. 6 includes a first low-pass filter formed with a resistor 33 and a capacitor 34 for removing noise components contained in an input signal from a rotational speed sensor 32, a second low-pass filter having a resistor 35 and a capacitor 36 producing a reference signal for detecting a voltage level developed by a constant current supplied from a 5V constant current source 37, operational amplifiers 38 and 39 for changing the noise-removed signal and the reference signal in impedance, a threshold determining circuit built with constant current sources 42, 43 and resistors 40 and 41 for providing high and low threshold values for shaping the output from the operational amplifiers 38 and 39, comparators 44 and 45, and an R-S flip-flop 47 for producing a wave-shaped signal based on the high and low threshold values. The wave-shaping circuit further includes a comparator 46 which monitors the reference signal for detecting a circuit failure such as wire breakage of the rotational speed sensor 32 and provides a failure indicative signal from a terminal f.sub.NG to have an external device (not shown) indicate occurrence of the circuit failure.
In the above conventional wave-shaping circuit, an input signal from the rotational speed sensor 32 is adjusted by the current supplied from the constant current source and the activities of the resistor to lie over an in-phase input voltage range defined by the high and low threshold values, and the threshold values are modified so as to follow the variation in center of amplitude of the output signal from the rotational speed sensor 32. However, since the variation in voltage of the input signal may be developed due to variation in resistance value of the rotational speed sensor, a complex pulse-shaping circuit is needed. In other words, the conventional wave-shaping circuit requires the low-pass filter for producing the reference signal serving to detect a voltage level pulled up by the constant current source. Additionally, since the reference signal is unstable, the additional constant current sources 42 and 43 and the resistors 40 and 41 are also required for stability of the reference signal in order to determine the high and low threshold values based on the reference signal. Further, since these constant current sources and resistors generally have temperature-dependent characteristics, a temperature-compensating circuit is required which compensates for the variations in the characteristics caused by variation in ambient temperature.